Micromodule cables and breakout cables therefor

ABSTRACT

A breakout cable includes a polymer jacket and a plurality of micromodules enclosed within the jacket. Each micromodule has a plurality of bend resistant optical fibers and a polymer sheath comprising PVC surrounding the bend resistant optical fibers. Each of the plurality of bend resistant optical fibers is a multimode optical fiber including a glass cladding region surrounding and directly adjacent to a glass core region. The core region is a graded-index glass core region, where the refractive index of the core region has a profile having a parabolic or substantially curved shape. The cladding includes a first annular portion having a lesser refractive index relative to a second annular portion of the cladding. The first annular portion is interior to the second annular portion. The cladding is surrounded by a low modulus primary coating and a high modulus secondary coating.

PRIORITY APPLICATIONS

This application is a continuation of U.S. application Ser. No.12/705,739, filed Feb. 15, 2010, which claims the benefit of U.S.application Ser. No. 61/152,860, filed Feb. 16, 2009, the entirecontents of each of which are hereby incorporated by reference as ifpresented herein.

TECHNICAL FIELD

The present disclosure relates to an optical cable with micromodules andthe use of micromodule cables.

BACKGROUND

Micromodule cables are high density optical cables having a numberindividual micromodules. The micromodules typically include opticalfibers arranged in a sheath for transmitting optical data. Themicromodules are arranged within the cable jacket. Conventionalmicromodule cables are capable of transmitting large amounts of data andare typically designed for outdoor applications.

One of the problems in planning data centers is the large number ofcables required to transfer data between racks of electronic equipment.The cables may become tightly packed, which restricts cooling air flowin and around the equipment. Micromodule cables have the ability totransmit large amounts of data, but existing micromodule cables lackmany properties that render them suitable for applications such as datacenters. For example, prior art high density cables have used relativelyflimsy subunit materials such as chalk-filled EVA or polyester-basedthermoplastic elastomers. The subunits have had wall thicknesses ofabout 0.1 mm, which provided only minimum protection for the opticalfibers. Furcation of these cables required that the subunits be placedwithin protective furcation legs so that they were robust enough forfield use. Existing micromodule cables may also lack the ability to bebent around corners or other tight spaces without undue attenuation ofthe optical data. Further, conventional micromodule cables may not bedesigned to pass burn specifications such as plenum-ratings.

SUMMARY

According to one embodiment, a breakout cable comprises a jacket and aplurality of micromodules enclosed within the jacket, each micromodulehaving a plurality of optical waveguides and a polymer sheathsurrounding the waveguides. The polymer sheath has a thickness in therange of 0.2 mm and 0.3 mm. The breakout cable may have exceptionallylow attenuation under various test criteria, such as when subjected to acorner bend load of six kilograms, delta attenuation at 850 nm in thecable is less than 0.2 dB. The polymer sheath surrounding the waveguidescan allow access to the optical waveguides by hand tearing, while beingrobust enough so that they can serve as furcation legs. According to oneaspect, the polymer sheath has a thickness in the range of 0.2 mm-0.3 mmand still allows for hand access to the optical waveguides.

According to another embodiment, a breakout cable includes a polymerjacket and a plurality of micromodules enclosed within the jacket. Eachmicromodule has a plurality of bend resistant optical fibers and apolymer sheath comprising PVC surrounding the bend resistant opticalfibers. Each of the plurality of bend resistant optical fibers is amultimode optical fiber including a glass cladding region surroundingand directly adjacent to a glass core region. The core region is agraded-index glass core region, where the refractive index of the coreregion has a profile having a parabolic or substantially curved shape.The cladding includes a first annular portion having a lesser refractiveindex relative to a second annular portion of the cladding. The firstannular portion is interior to the second annular portion. The claddingis surrounded by a low modulus primary coating and a high modulussecondary coating.

According to another embodiment, a plurality of the breakout cables areenclosed within an outer jacket to form a micromodule breakout cable.

According to one aspect, a method of calculating jacket thickness for acable is used to provide a breakout cable with robustness, compliancewith plenum burn requirements, and with hand accessibility. The methoddetermines a maximum jacket thickness based on material modulus.

Those skilled in the art will appreciate the above stated advantages andother advantages and benefits of various additional embodiments readingthe following detailed description of the embodiments with reference tothe below-listed drawing figures.

BRIEF DESCRIPTION OF THE FIGURES

The present embodiments are explained in more detail below withreference to figures which show the exemplary embodiments.

FIG. 1 is a cross section of a micromodule cable according to a firstembodiment.

FIG. 2 is a sectional view of a breakout unit or cable of the cable ofFIG. 1.

FIG. 3 is a sectional view of a micromodule of the cable of FIG. 1.

FIG. 4 is a sectional view of a cable according to a second embodiment.

FIG. 5 is a plot illustrating various design constraints in designing acable for strength, accessibility and for plenum burn characteristics.

FIG. 6 is a schematic representation (not to scale) of the refractiveindex profile of a cross-section of the glass portion of an exemplaryembodiment of multimode optical fiber disclosed herein wherein thedepressed-index annular portion is offset from the core and issurrounded by an outer annular portion.

FIG. 7 is a schematic representation (not to scale) of a cross-sectionalview of the optical waveguide fiber of FIG. 6.

FIG. 8 illustrates a cable according to a third embodiment undergoing acorner tie down bend.

FIG. 9 is a plot of delta attenuation at 850 nm for the cable of FIG. 8in a corner tie down bend test.

FIG. 10 is a plot of delta attenuation in a corner bend under load test.

FIG. 11 is a plot of delta attenuation for one embodiment of theinvention in small diameter mandrel wrap tests.

DETAILED DESCRIPTION

FIG. 1 is a cross section of an optical micromodule cable 10 accordingto a first embodiment and having a generally circular cross-section. Theoptical cable 10 comprises a plurality of breakout units or cables 20arranged (e.g. stranded) around a central member 30 in an interior 34 ofthe cable 10. The central member 30 can be, for example, a relativelystiff member 36 of fiber or glass-reinforced plastic, or a relativelyflexible combination of aramid fiber that may include an overcoating ofplastic material 38. The breakout units 20 are enclosed within thecable's outer jacket 50. The outer jacket 50 can be formed from, forexample, a polymer material, and may be reinforced with fibers, etc.(not shown), and has a thickness 52. Each breakout unit 20 includes aplurality of micromodule subunits 60, or simply “micromodules”, thateach contain at least one optical waveguide 66. The exemplarymicromodules 60 are not stranded within the breakout units 20, althoughstranding may be used for certain applications. For example, themicromodules 60 can be twisted in helical fashion with respect to oneanother, in particular when a plurality of or all of the micromodules 60are arranged in such a way that they are rotated with a specified laylength.

In the illustrated embodiment, the micromodule breakout cable 10 haseight breakout units 20, each breakout unit 20 having twelvemicromodules 60, with each micromodule 60 including twelve opticalwaveguides 66. The total number of optical waveguides 66 for the cableis therefore 1152. Other numbers of breakout units 20, micromodules 60,and optical waveguides 66 can be employed for various applications,however. The micromodule cable 10, the breakout units 20 and themicromodules 60 all have generally circular cross-sections, althoughother cross-sections may be used.

FIG. 2 is a cross section of one of the exemplary breakout units 20having diameter 22. Each breakout unit 20 has a jacket 70 of thickness72 that encloses its micromodules 60. The thickness 72 will not becompletely uniform and thicknesses, as well as diameters described inthis specification refer to nominal or average values. A strain-reliefelement 76 may be disposed adjacent to the interior of the jacket 70 andsurrounding the micromodules 60. The strain-relief element 76 may be,for example, a layer of yarns (e.g. aramid yarn) for absorbing tensileloads on the cable 10. In FIG. 2, the exemplary strain-relief element is76 are illustrated schematically as a layer of yarn disposed adjacent tothe interior of outer jacket 70. The layer of strain-relief element 76is shown with a non-uniform thickness because the locations of themicromodules 60 may cause the strain-relief element to compress atvarious locations along the length of the breakout unit 20.

FIG. 3 is a cross section of a micromodule subunit 60 having a diameter62. The optical waveguides 66 of the micromodules 60 are enclosed in apolymeric sheath 80 of thickness 82.

According to one aspect of the first embodiment, the micromodule cable10 can be constructed to achieve desired properties. For example, thecomponents of the cable 10 can be constructed of selected materials ofselected thicknesses such that the cable 10 achieves plenum burn ratingsaccording to desired specifications. The micromodules 60 can also beconstructed so that they are relatively robust, such that they aresuitable for field use, while also providing a desired degree ofaccessibility. For example, the micromodules 60 according to the presentembodiments can be constructed with thicker sheaths 80, on the order of0.2 mm or more, which provide sufficient protection for the fibers suchthat the micromodules 60 may be used as a furcation leg. The cablejacket 50, the breakout unit jackets 70, and the micromodule sheaths 80can also be formed from fire-retardant materials to obtain a desiredplenum burn rating. For example, highly-filled PVC of a specifiedthicknesses can be used to form the micromodule sheaths 80. One wellknown plenum burn standard is the National Fire Protection Act Standards(NFPA) 262 burn test. NFPA 262 prescribes the methodology to measureflame travel distance and optical density of smoke for insulated,jacketed, or both, electrical wires and cables and optical fiber cablesthat are to be installed in plenums and other spaces used to transportenvironmental air without being enclosed in raceways. Cables accordingto the present embodiments may also be constructed to be low skew withinthe micromodules 60 so that they are suitable for use in parallel optictransmission systems. Skew is generally defined as the difference in thetime of flight of optical signals for the fibers within a module and hasunits of picoseconds per meter (ps/m).

FIG. 4 is a section view of a cable 200 according to a second embodimenthaving a diameter 202. The cable 200 is generally similar to thebreakout units 20 of the cable 10, but the cable 200 may have a jacket270 of greater thickness 272 than the thickness 72 of the breakout unitjacket 70 to provided added robustness to the cable 200. The interior ofthe cable 200 may also allow for greater spacing of the micromodules260. In FIG. 4, elements of like or identical construction to those ofFIGS. 1-3 are indicated with like reference numbers, with the elementsin FIG. 4 being preceded by a “2.” The micromodules 260 illustrated inFIG. 4 may, for example, be identical to the micromodules 60 illustratedin FIGS. 1-3. As in the case of the cable 10, the cable 200 can beconstructed to have desirable burn properties as well as a selecteddegree of durability and hand accessibility. Exemplary methodologies forconstructing cables according to the present embodiments are discussedin detail below.

The following discussion is addressed to the components of the cable 200illustrated in FIG. 4, although the principles discussed herein apply tothe cable 10 (FIGS. 1-3). The micromodule sheath 280 thickness 282 maybe increased or decreased to adjust the properties of the cable 200. Forexample, the micromodules 260 may be made with a thicker sheath 280 tomake a more robust unit, or it may be made with a thinner sheath wall inorder to reduce size and material costs. However, according to thepresent embodiments, additional constraints may be placed on thematerials and dimension of the micromodule sheath 280 thickness. Onedesirable property is accessibility, or the ability to easily remove themicromodule sheath 80 from around the optical waveguides 66 by simpletearing with the fingers. The axial strength of the micromodule sheath280 is the cross sectional area of the sheath 280 times the modulus ofthe material. A plenum grade PVC was tested as the micromodule sheath280 material having a modulus of 2500 psi with sheath thicknesses of 0.1mm, 0.2 mm, and 0.3 mm. A sheath 280 thickness of 0.2 mm allowed foreasy removal of the sheath 280, but at 0.3 mm it became much moredifficult to remove the sheath. A micromodule sheath 280 of thickness inthe range of 0.2 mm to 0.3 mm accordingly provides highly desirableproperties not available in existing cables. Using these values asdesign constraints, the maximum sheath thickness for a given materialmodulus may be calculated using the following Equation 1 (wall thicknessin millimeters and modulus in psi):Material_Modulus=590(Wall_thickness)^(−1.2).

The constraints discussed above define a desired operating region. TheDESIRED OPERATING REGION is represented on a plot of wall thicknessversus modulus as shown in FIG. 5.

The methodologies set forth above may be applied to alternative forms ofpackaging. Equation 1 establishes a relationship and practical limit forany packaging that is intended for easy opening by hand. For example, itcould be used in the design of a vessel accommodating foodstuffs. InFIG. 5, the minimum wall (or sheath) thickness is determined by theplenum burn test; however, for a bag of foodstuffs, the minimum wallthickness can be determined by the minimum thickness to maintainfreshness or some other constraint. Another direct application ofEquation 1 is in the design of packaging for emergency medical equipmentin which the package must maintain a sterile environment inside but beeasily opened by the medical technician during an emergency. Equation 1can be used to determine the maximum wall thickness, and the minimumwall thickness can be determined by the thickness required to maintainthe contents in a sterile environment.

According to another aspect of the present embodiments, the cables 10,200 may be used in data center applications, such as in a datainformation transfer system. In data centers, optical signals aretransmitted and received in blade servers. Common blade servers havefrom 16 to 48 optical ports. Each optical port has a transmit fiber anda receive fiber. In parallel optic systems, the transmit fiber would bereplaced by 12 fibers and the receive fiber would be replaced by 12fibers in a 120 Gb/s Ethernet blade server. Thus a parallel optic systemwith a 48 port blade servers would need 1152 optical fibers. An examplecable suitable for use in a data center is described below:

Example 1

According to one example embodiment, a cable 10 as shown in FIG. 1includes twelve optical waveguides 66, in the form of optical fibers, ineach micromodule 60. The micromodule diameter 62 is 1.6 mm. Twelvemicromodules 60 are placed together in a 144 fiber breakout unit 20having a diameter 22 of about 8.6 mm. Eight breakout units 20 are placedtogether in an 1152 fiber cable that is less than 36 mm in diameter 12.

According to the above example, if desired, the fiber count can beincreased to 1728 fibers in a plenum cable of less than 40 mm indiameter by using twelve breakout units stranded in a 9-over-3 pattern.

According to the above embodiments, the cable 200 may comply with NFPA262 while having a diameter 202 of less than 13.1 mm for a 144 fibercable. The cables 10, 200 may have low skew for parallel optic systemsof less than 2.0 ps/m.

In the above embodiments, low skew may be obtained in the subunits byusing OM3 grade or better multimode fibers and precise control of thefiber tension during processing. The maximum allowed difference in fibertension during processing of the subunit should be less than 50 g inorder to achieve a skew less than 0.75 ns in a 300 meter cable assembly.The numerical aperture (NA) of the optical fiber should also becontrolled. The fibers for the micromodules can be selected so that thedifference in NA for the fibers within a subunit are less than 0.08 toachieve a skew less than 0.75 ns in a 300 m cable assembly.

One exemplary fiber suitable for use in the above cables is a bendresistant multimode optical fibers comprising a graded-index core regionand a cladding region surrounding and directly adjacent to the coreregion, the cladding region comprising a depressed-index annular portioncomprising a depressed relative refractive index relative to anotherportion of the cladding. The depressed-index annular portion of thecladding is preferably spaced apart from the core. Preferably, therefractive index profile of the core has a parabolic or substantiallycurved shape. The depressed-index annular portion may, for example,comprise a) glass comprising a plurality of voids, or b) glass dopedwith one or more downdopants such as fluorine, boron, individually ormixtures thereof The depressed-index annular portion may have arefractive index delta less than about −0.2% and a width of at leastabout 1 micron, said depressed-index annular portion being spaced fromsaid core by at least about 0.5 microns. In some embodiments thatcomprise a cladding with voids, the voids in some preferred embodimentsare non-periodically located within the depressed-index annular portion.By “non-periodically located” we mean that when one takes a crosssection (such as a cross section perpendicular to the longitudinal axis)of the optical fiber, the non-periodically disposed voids are randomlyor non-periodically distributed across a portion of the fiber (e.g.within the depressed-index annular region). Similar cross sections takenat different points along the length of the fiber will reveal differentrandomly distributed cross-sectional hole patterns, i.e., various crosssections will have different hole patterns, wherein the distributions ofvoids and sizes of voids do not exactly match for each such crosssection. That is, the voids are non-periodic, i.e., they are notperiodically disposed within the fiber structure. These voids arestretched (elongated) along the length (i.e. generally parallel to thelongitudinal axis) of the optical fiber, but do not extend the entirelength of the entire fiber for typical lengths of transmission fiber. Itis believed that the voids extend along the length of the fiber adistance less than about 20 meters, more preferably less than about 10meters, even more preferably less than about 5 meters, and in someembodiments less than 1 meter. The multimode optical fiber disclosedherein exhibits very low bend induced attenuation, in particular verylow macrobending induced attenuation. In some embodiments, highbandwidth is provided by low maximum relative refractive index in thecore, and low bend losses are also provided. Consequently, the multimodeoptical fiber may comprise a graded index glass core; and an innercladding surrounding and in contact with the core, and a second claddingcomprising a depressed-index annular portion surrounding the innercladding, said depressed-index annular portion having a refractive indexdelta less than about −0.2% and a width of at least 1 micron, whereinthe width of said inner cladding is at least about 0.5 microns and thefiber further exhibits a 1 turn, 10 mm diameter mandrel wrap attenuationincrease of less than or equal to about 0.4 dB/turn at 850 nm, anumerical aperture of greater than 0.14, more preferably greater than0.17, even more preferably greater than 0.18, and most preferablygreater than 0.185, and an overfilled bandwidth greater than 1.5 GHz-kmat 850 nm. 50 micron diameter core multimode fibers can be made whichprovide (a) an overfilled (OFL) bandwidth of greater than 1.5 GHz-km,more preferably greater than 2.0 GHz-km, even more preferably greaterthan 3.0 GHz-km, and most preferably greater than 4.0 GHz-km at an 850nm wavelength . These high bandwidths can be achieved while stillmaintaining a 1 turn, 10 mm diameter mandrel wrap attenuation increaseat an 850 nm wavelength of less than 0.5 dB, more preferably less than0.3 dB, even more preferably less than 0.2 dB, and most preferably lessthan 0.15 dB. These high bandwidths can also be achieved while alsomaintaining a 1 turn, 20 mm diameter mandrel wrap attenuation increaseat an 850 nm wavelength of less than 0.2 dB, more preferably less than0.1 dB, and most preferably less than 0.05 dB, and a 1 turn, 15 mmdiameter mandrel wrap attenuation increase at an 850 nm wavelength, ofless than 0.2 dB, preferably less than 0.1 dB, and more preferably lessthan 0.05 dB. Such fibers are further capable of providing a numericalaperture (NA) greater than 0.17, more preferably greater than 0.18, andmost preferably greater than 0.185. Such fibers are furthersimultaneously capable of exhibiting an OFL bandwidth at 1300 nm whichis greater than about 500 MHz-km, more preferably greater than about 600MHz-km, even more preferably greater than about 700 MHz-km. Such fibersare further simultaneously capable of exhibiting minimum calculatedeffective modal bandwidth (Min EMBc) bandwidth of greater than about 1.5MHz-km, more preferably greater than about 1.8 MHz-km and mostpreferably greater than about 2.0 MHz-km at 850 nm. Preferably, themultimode optical fiber disclosed herein exhibits a spectral attenuationof less than 3 dB/km at 850 nm, preferably less than 2.5 dB/km at 850nm, even more preferably less than 2.4 dB/km at 850 nm and still morepreferably less than 2.3 dB/km at 850 nm. Preferably, the multimodeoptical fiber disclosed herein exhibits a spectral attenuation of lessthan 1.0 dB/km at 1300 nm, preferably less than 0.8 dB/km at 1300 nm,even more preferably less than 0.6 dB/km at 1300 nm. In someembodiments, the numerical aperture (“NA”) of the optical fiber ispreferably less than 0.23 and greater than 0.17, more preferably greaterthan 0.18, and most preferably less than 0.215 and greater than 0.185.In some embodiments, the core extends radially outwardly from thecenterline to a radius R1, wherein 10≦R1≦40 microns, more preferably20≦R1≦40 microns. In some embodiments, 22≦R1≦34 microns. In somepreferred embodiments, the outer radius of the core is between about 22to 28 microns. In some other preferred embodiments, the outer radius ofthe core is between about 28 to 34 microns. In some embodiments, thecore has a maximum relative refractive index, less than or equal to 1.2%and greater than 0.5%, more preferably greater than 0.8%. In otherembodiments, the core has a maximum relative refractive index, less thanor equal to 1.1% and greater than 0.9%. In some embodiments, the opticalfiber exhibits a 1 turn, 10 mm diameter mandrel attenuation increase ofno more than 1.0 dB, preferably no more than 0.6 dB, more preferably nomore than 0.4 dB, even more preferably no more than 0.2 dB, and stillmore preferably no more than 0.1 dB, at all wavelengths between 800 and1400 nm.

FIG. 6 is a schematic representation of the refractive index profile ofa cross-section of the glass portion of an embodiment of a multimodeoptical fiber 400 comprising a glass core 420 and a glass cladding 500,the cladding comprising an inner annular portion 530, a depressed-indexannular portion 550, and an outer annular portion 560. FIG. 7 is aschematic representation (not to scale) of a cross-sectional view of theoptical waveguide fiber of FIG. 6. The core 420 has outer radius R1 andmaximum refractive index delta Δ1MAX. The inner annular portion 530 haswidth W2 and outer radius R2. Depressed-index annular portion 550 hasminimum refractive index delta percent Δ3MIN, width W3 and outer radiusR3. The depressed-index annular portion 550 is shown offset, or spacedaway, from the core 420 by the inner annular portion 530. The annularportion 550 surrounds and contacts the inner annular portion 530. Theouter annular portion 560 surrounds and contacts the annular portion550. The clad layer 500 is surrounded by at least one coating 510, whichmay in some embodiments comprise a low modulus primary coating and ahigh modulus secondary coating. The inner annular portion 530 has arefractive index profile Δ2(r) with a maximum relative refractive indexΔ2MAX, and a minimum relative refractive index Δ2MIN, where in someembodiments Δ2MAX=Δ2MIN. The depressed-index annular portion 550 has arefractive index profile Δ3(r) with a minimum relative refractive indexΔ3MIN. The outer annular portion 560 has a refractive index profileΔ4(r) with a maximum relative refractive index Δ4MAX, and a minimumrelative refractive index Δ4MIN, where in some embodiments Δ4MAX=Δ4MIN.Preferably, Δ1MAX>Δ2MAX>Δ3MIN. In some embodiments, the inner annularportion 530 has a substantially constant refractive index profile, asshown in FIG. 6 with a constant Δ2(r); in some of these embodiments,Δ2(r)=0%. In some embodiments, the outer annular portion 560 has asubstantially constant refractive index profile, as shown in FIG. 6 witha constant Δ4(r); in some of these embodiments, Δ4(r)=0%. The core 420has an entirely positive refractive index profile, where Δ1(r)>0%. R1 isdefined as the radius at which the refractive index delta of the corefirst reaches value of 0.05%, going radially outwardly from thecenterline. Preferably, the core 420 contains substantially no fluorine,and more preferably the core 420 contains no fluorine. In someembodiments, the inner annular portion 530 preferably has a relativerefractive index profile Δ2(r) having a maximum absolute magnitude lessthan 0.05%, and Δ2MAX<0.05% and Δ2MIN>−0.05%, and the depressed-indexannular portion 550 begins where the relative refractive index of thecladding first reaches a value of less than −0.05%, going radiallyoutwardly from the centerline. In some embodiments, the outer annularportion 560 has a relative refractive index profile Δ4(r) having amaximum absolute magnitude less than 0.05%, and Δ4MAX<0.05% andΔ4MIN>−0.05%, and the depressed-index annular portion 550 ends where therelative refractive index of the cladding first reaches a value ofgreater than −0.05%, going radially outwardly from the radius whereΔ3MIN is found.

FIG. 8 illustrates a cable 600 according to a third embodiment of thepresent invention tied down to experience a 90 degree corner bend ofapproximately three-fourths inch (19 mm) radius. The cable 600 can betied down to experience any number of corner bends. The cable 600 isgenerally similar to the cable 200 illustrated in FIG. 4, andincorporates 48 optical fibers of a configuration as discussed in theimmediately preceding paragraphs. The cable 600 has a jacket 650, andincludes 4 micromodules, each micromodule having 12 fibers.

FIG. 9 is a plot of corner tie down delta attenuation data at 850 nm forthe cable 600 undergoing varying numbers of corner bends, as shown inFIG. 8, of approximately one inch (25.4 mm) radius. Measured data pointsfor the cable 600, of relatively low attenuation, are generallyindicated by reference numeral 655. For reference purposes, higher deltaattenuation data for similar cables utilizing alternative multimodefibers are also illustrated. As shown in FIG. 9, the delta attenuationvalues in group 655 over two, four, six and even eight bends are below0.05 dB. More specifically, delta attenuation values for each of thedata values in group 655 shown in FIG. 9 are below 0.05 dB, and even aslow as 0.03 dB or less.

Referring now to FIG. 10, the cable 600 can also be subjected to acorner bend under load. In a corner bend under load test, a weight ishung from an end of the cable that hangs over the corner of a surface.The corner may have a small radius, such as 1 mm. The other end of thecable is secured on the surface. Test delta attenuation data for thecorner bend under load are shown in FIG. 10. The delta attenuation data,generally indicated by the bracket 660, for the cable 600 was much lowerthan similar cables utilizing alternative fibers. For loads of up to 8kg, delta attenuation was less than about 0.05 dB. For loads of up to 6kb, delta attenuation was less than about 0.03 dB.

FIG. 11 is a plot of minimum bend radius test data for the cable 600that contains twelve bend insensitive multimode optical fibers and hasan outer diameter of 4.4 mm. The standard cable bend requirements isthat the cable have low attenuation when wrapped around a mandrel thatis ten times the cable outside diameter. FIG. 11 demonstrates that thecable 600 is capable of achieving bend diameters much smaller than thecurrent 10× standard by having low attenuation at five times and even aslow as three times the cable diameter. For example, for five wrapsaround a mandrel having a diameter of five times the cable diameter,delta attenuation due to the wraps is less than 0.15 dB. For one wraparound a mandrel having a diameter of three times the cable diameter,delta attenuation due to the wraps is less than 0.10 dB.

The present cable embodiments may utilize tensile yarns to form aseparation layer between the modules and the outer jacket and thusprevent the modules from sticking to the jacket during extrusion of thejacket. The tensile yarns also provide strength to the cables. Apreferred material for the tensile yarns is aramid (e.g., KEVLAR®), butother materials such as fiberglass yarn may also be used. The yarns maybe stranded to improve cable performance. In the illustrated cables, thejackets may be sized such that the micromodules have a sufficient degreeof lateral movement to reduce fiber stresses and optical attenuationwhen the cable is exposed to external forces such as tension, torsion,bending, or compression. The void fraction within the cable jackets maybe about 56%.

In one particular set of parameters, cables according to the presentembodiments may contain from four to twelve optical fibers within eachmicromodule. The breakout units 20 or the cables 200, 600 may containfrom 2 to 24 micromodules within the cable for a range of fiber countsof 8 to 288. The dimensions of the micromodule may be adjusted based onthe number of fibers within the module. The fibers may be looselydisposed within the module in an essentially parallel array. The fibersmay be coated with a thin film of powder, such as chalk or talc, whichforms a separation layer that prevents the fibers from sticking to themolten sheath material during extrusion. The outer jackets may be madeof a fire retardant PVC material or a PVDF material to achieve a plenumburn performance rating. The cables may be further encased in aninterlocking armor for enhanced crush resistance.

Many modifications and other embodiments of the present invention,within the scope of the claims will be apparent to those skilled in theart. For instance, the concepts of the present invention can be usedwith any suitable fiber optic cable design and/or method of manufacture.For instance, the embodiments shown can include other suitable cablecomponents such as an armor layer, coupling elements, differentcross-sectional shapes, or the like. Thus, it is intended that thisinvention covers these modifications and embodiments as well those alsoapparent to those skilled in the art.

What is claimed is:
 1. A micromodule cable, comprising: an outer jacket;a plurality of breakout units enclosed in the outer jacket, eachbreakout unit having a plurality of micromodules, a jacket enclosing themicromodules, and an aramid strain relief element adjacent to thejacket, with each micromodule having at least one bend resistant opticalfiber and a polymer sheath surrounding the at least one bend resistantoptical fiber, wherein the at least one bend resistant optical fibercomprises a glass cladding region surrounding and directly adjacent to aglass core region, wherein the cladding comprises a first annularportion having a lesser refractive index relative to a second annularportion of the cladding, wherein the first annular portion is interiorto the second annular portion, and wherein the cladding is surrounded bya low modulus primary coating and a high modulus secondary coating; anda central strength member, the breakout units being arranged around thecentral strength member, wherein the cable satisfies the NFPA 262 plenumburn test, wherein each polymer sheath comprises polyvinyl chloride, andwherein when the cable is subjected to a corner bend tie down test attwo tie down bend locations of 25.4 mm radius, delta attenuation at 850nm in the cable due to the bend is less than 0.05 dB.
 2. The micromodulecable of claim 1, wherein the at least one bend resistant optical fiberin the micromodules can be accessed by tearing the micromodule sheathwith a user's fingers.
 3. The micromodule cable of claim 2, wherein thepolymer sheaths have a thickness in the range of 0.2 to 0.3 mm.
 4. Themicromodule cable of claim 1, wherein when subjected to a corner bendload of six kilograms, delta attenuation at 850 nm in the breakout unitsdue to the load is less than 0.4 dB.
 5. The micromodule cable of claim1, wherein the core region of the at least one optical fiber has a 50micron diameter which provides an overfilled bandwidth of greater than1.5 GHz-km.
 6. The micromodule cable of claim 5, wherein the core regioncontains substantially no fluorine.
 7. The micromodule cable of claim 1,wherein the polyvinyl chloride is a highly-filled polyvinyl chloride.